In dogs and cats food allergies tend to be to proteins. There needs to be prior exposure to the protein to be able to develop an allergy to it. Allergies to preservatives are seen as a totally different scenario in dogs especially and present in a manner much more closely to those signs seen with drug allergies.
Food allergies cannot reliably be tested for with blood tests. The best method is via a period of feeding a diet which has been determined to be hypoallergenic (something the pet hasn’t been exposed to before) and then rechallenging with individual proteins. When determining the hypoallergenic diet it is important that all regularly fed foods be considered, as many have ingredients, especially proteins in common, so merely changing brands will do little to achieve the end goal. Common hypoallergenic foods in Australia include fish and kangaroo as a protein source. Potato or tapioca are often used as a carbohydrate source. Overseas, lamb is commonly used, but in Australia lamb is widely eaten by most people and meat eating pets so it is not suitable.
Some foods maybe labelled as ‘for sensitive skin’. There are many different types of these, and many are not suitable for an initial diet trial. They often contain a single protein source, which is great if you have determined that protein source is safe for your pet, and may also contain added extras such as omegas. The omega 3 and 6 fatty acids are not overly useful as a sole therapy in skin disease management but do have some anti-inflammatory effects on the skin by inhibiting small sectors of the complex allergy pathway in the body, and can be helpful for some pets.